This invention relates to a method of monitoring a resistance spot welding process during the weld operation and to a multivariable process monitor to detect poor quality welds.
Most resistance spot welding production procedures consist of establishing the process welding parameters by trial and error. Once these production parameters have been established, the percent heat, electrode load and cycles are set. During production, occasional test coupons are made which are destructively inspected to determine weld nugget size and penetration. This manufacturing procedure has produced faulty welds since the test coupon geometry, surface conditions, etc., may not be representative of the production conditions. The intent of this invention is to eliminate the problem of poor welds during production by sensing dominant variables needed to assure weld quality.
There are essentially four types of spot welder process monitors on the market which are grouped according to the single parameter measured in the welding process. They are: (1) thermal expansion monitors; (2) nugget resistance monitors; (3) power input monitors; and (4) ultrasonic, acoustic, and infrared type monitors. It was concluded that no single system existing in the market can be relied on to give assurance of spot weld quality. Experiments have shown, for instance, that type (1) monitors based on measuring only thermal expansion can produce poor welds even though the system predicted good quality. The advantages of installing the disclosed improved process monitor on a resistance spot welder is that it will prevent poor welds from creeping into production without being detected and eliminate the costly and time consuming procedure of welding test coupons for destructive examination.